sizing 80% of breaker or MCC 1

[stodgyguy20]

I dont know if this is a NEC rule, but in this company most of the MCC or Bus or Breaker are only used up to 80% of their max or ratings.
For example, a 600A MCC is only used up to 480A.

My question is this:
1-why do most people use it up tp 80% only?

2-What will happen if I used it up to 90% or even 100% of its rating?

 

[davidbeach]

Time for you to do some code research.

1 - 'cause they have to.
2 - bad things.

 

[DRWeig]

NEC, several different places, requires ampacity to be 125% of continuous load. There are exceptions for equipment that is listed for continuous duty at 100%, but I don't remember any such exceptions for motor loads -- except for multi-motor equipment, in which case you have to start with 125% of the largest motor and add 100% of the rest.

Perhaps someone else will chime in with specific references.

You're not permitted to install new loads that will exceed the rating. It probably won't start a fire, but the code prohibits you from trying it. The headroom is to cover unforeseeable thermal effects and to prevent excessive voltage drop.

Good on ya,

Goober Dave

 

[tem1234]

The attached document should answer your question

 

[sspeare]

See Article 210.20, Overcurrent Protection, in the 2008 National Electric Code. If the breaker is "standard rated", then you have to apply the 80% loading rule for a continuous load. If the breaker is "fully rated" you can use the breaker at it's full nameplate rating.

 

[stodgyguy20]

I think I did not ask the questions the way I should have but Sspeare did provide.
Basically, I just wanted to know what happens if you use more that 80% for standard rated. it is just out of curiosity since I haven't seen it

 

[stodgyguy20]

also thanks DRWeig & tem1234 for your replies. you made it more clear to me.

davidbeach lol at "Time for you to do some code research". I've seen the required 80% but I just wanted to know exactly what happened if you go slightly above it

 

[rbulsara]

If you exceed 80% rating for any length of time, the breaker/fuse, or enclosure may overheat and even false trip. This is mainly due to fact that mfrs. have taken (dis)advantage of the code's safety margin while designing their device or panel/switchboard assemblies when for cooling effectiveness/ ambient temperature rise.

On the other hand, for example, if you have only one of the breakers in a group mounted assembly exceed 80%, but rest of the devices are below 80%, nothing may happen. But no guarantees.

Rafiq Bulsara

http://www.srengineersct.com

 

[wcarter]

stodgyguy20,

The issue is usually seen with thermal-mag breakers. It depends wether they are designed to operate in a open air or closed section. A good explaination to this is:

http://ecmweb.com/mag/electric_sizing_circuit_breaker/

It might be a bit clearer in how it states everything. But to sum it up, in open air, you have more airflow. Therefore, more cooling. In an enclosed structure, less to no airflow, plus surrounding metal. There will be a higher build-up of heat. This can cause the breaker to trip on its thermal protection earlier than desired.

In breakers that have CT's and electronic trip units, this is not a concern due to the current, not the heat being measured.

 

[jraef]

Breakers are actually made to carry 100% of the rating, but only in free air. The conductors feeding and coming from them are not allowed to be loaded beyond 80% under general conditions. There are rules that allow 100% rating under very specific circumstances. If used at 100%, the breaker must have a label saying it is being used that way. For most breaker mfrs that's a different breaker or option. Sometimes the difference is just in using 90C rated lugs, because to get 100% from a conductor you must use 90C insulation at 75C ratings (among other things). But nothing about this can be universally applied, the rules for 100% are strict and rarely used.

Bus bars don't have that restriction, but they are protected by breakers and fed by conductors that do.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

Breakers are actually made to carry 100% of the rating, but only in free air. The conductors feeding and coming from them are not allowed to be loaded beyond 80% under general conditions. There are rules that allow 100% rating under very specific circumstances. If used at 100%, the breaker must have a label saying it is being used that way. For most breaker mfrs that's a different breaker or option. Sometimes the difference is just in using 90C rated lugs, because to get 100% from a conductor you must use 90C insulation at 75C ratings (among other things). But nothing about this can be universally applied, the rules for 100% are strict and rarely used.

Bus bars don't have that restriction, but they are protected by breakers and fed by conductors, that do.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376